Rubber recycling manages the massive volume of discarded rubber products, primarily end-of-life vehicle tires. Over one billion tires are discarded globally each year, making effective recycling environmentally pressing. The difficulty in repurposing this material stems from vulcanization, a chemical treatment that gives rubber durability and elasticity by forming stable sulfur cross-links. This network prevents the rubber from being easily melted and reshaped like thermoplastics, requiring specialized mechanical, chemical, or thermal processes to recover its value.
Mechanical Processing and Crumb Rubber Production
The initial and most widely adopted method for rubber reclamation begins with mechanical size reduction, ultimately producing a material known as crumb rubber. The process starts with collecting and cleaning the scrap tires, which are then passed through powerful shredders that break them down into pieces often called chips or shreds. These rubber chips are then fed into granulators or cracker mills for further reduction.
Mechanical grinding occurs either at ambient temperatures or through a cryogenic process where the rubber is cooled using liquid nitrogen until it becomes brittle. Cryogenic grinding produces a finer, smoother particle, while ambient grinding is more common and cost-effective for larger particles. During this grinding phase, specialized equipment systematically separates the non-rubber components, primarily the reinforcing steel belts and textile fibers.
Powerful magnets remove the steel content, and wind sifters remove the lighter fiber material. The clean rubber granules are then sorted and graded into various sizes, measured in mesh count or millimeters. Sizes range from 3/8-inch particles suitable for general applications down to 40 mesh or smaller for high-specification uses.
Chemical and Thermal Recycling Techniques
When simple size reduction is insufficient, more advanced techniques are employed to break down the highly stable vulcanized structure. Devulcanization is a chemical or thermomechanical process designed to selectively cleave the sulfur cross-links without significantly damaging the main hydrocarbon polymer chains. This restoration of plasticity allows the rubber to be re-molded and compounded with virgin materials.
The devulcanization process involves heating ground rubber to temperatures from 80°C to 300°C under high pressure, sometimes with chemical agents. These agents, such as organic disulfides or supercritical carbon dioxide, act as catalysts to assist in the controlled scission of the C–S and S–S bonds. Careful control is necessary because the energy required to break the sulfur bonds is close to that needed to break the main C–C polymer chain, which would degrade the material.
Another sophisticated method is pyrolysis, a form of thermal decomposition that heats the rubber in a reactor vessel in the complete absence of oxygen. This oxygen-free environment, maintained at temperatures between 300°C and 600°C, prevents combustion and instead causes the long-chain rubber polymers to break down into three recoverable products. The primary outputs are a liquid known as pyrolytic oil, a solid residue called char, and non-condensable gases.
Pyrolytic oil, which can account for over 50% of the mass, is a blend of hydrocarbons used as a fuel source or refined into specialty chemicals. The solid char, making up approximately 35% of the output, contains recovered carbon black, inorganic fillers, and residual steel. The remaining gas fraction is often used to power the pyrolysis process itself, improving the energy efficiency of the operation.
Applications for Repurposed Rubber
The mechanically produced crumb rubber is widely used in applications where its physical properties, such as shock absorption and durability, are beneficial. Coarser grades are frequently molded into products like speed bumps, livestock mats, and flooring tiles for industrial or residential use. Fine crumb rubber is the primary material used as infill for artificial turf athletic fields and is mixed with binders to create poured-in-place safety surfacing for playgrounds.
Fine crumb rubber is also incorporated into asphalt pavement to produce rubberized asphalt concrete, which improves road durability and reduces traffic noise. Rubber that has been successfully devulcanized regains enough of its original elasticity to be blended with new rubber compounds. This reclaimed rubber can then be used in the manufacturing of new tires, hoses, and various industrial components, directly replacing a portion of virgin rubber.
The byproducts from thermal processing find utility in various industries. Pyrolytic oil is commonly used as a fuel in industrial boilers or kilns, or it can be fractionated into lighter fuel oils. The recovered carbon black from the char residue is processed and used as a reinforcing filler or pigment in new rubber and plastic products, closing the material loop.